1. Field of the Invention
This invention relates to a temperature information detecting device for an angle sensor and a position detecting device, more particularly to a temperature information detecting device for an angle sensor suitable for control of valve opening of a flow control valve and to a position detecting device using the temperature information detecting device.
2. Description of the Related Art
Conventionally, it has been made a habit in a flow control valve of detecting its valve opening by an angle sensor, of calculating the amount of control of the valve opening by the body of a positioner, a valve opening control device, based on the detected valve opening and a valve opening setting value given from the outside, and of automatically controlling the valve opening in order that it coincides with the valve opening setting value in accordance with the amount of control.
FIG. 3 is a block diagram showing the conventional flow control device.
Referring to FIG. 3 reference numeral 51 denotes a flow control valve, 52 denotes a yoke which fixes an air actuator 53 to the flow control valve 51, 54 denotes a valve driving shaft driven by the air actuator 53, 55 denotes a projecting pin provided at a predetermined position of the valve driving shaft 54, 56 denotes an angle sensor fixed to part of the yoke 52 for outputting a position signal in accordance with the valve opening, i.e., a valve position. Such angle sensor disclosed in Publication of Unexamined Japanese Patent Application No. 11-83422 has been placed in service, in which the angle sensor 56 is configured by combining a number of magnetic resistive elements into a bridge circuit, and input voltage is applied to two terminals facing each other, whereas output voltage is outputted from two other terminals facing each other.
Reference numeral 57 denotes a feedback lever for inputting to the angle sensor 56 opening information in accordance with the valve position of the flow control valve 51, whose one end is fixed to the axis of rotation of the angle sensor 56. The feedback lever 57 has a slit 57a formed thereon and the pin 55 is slidably engaged with the slit 57a for converting reciprocal motion of the valve driving shaft 54 into rotational motion of the angle sensor 56. Reference numeral 58 denotes the body of a positioner, the valve opening control device, which is housed together with the angle sensor 56 in a case fixed to the yoke 52, 59 denotes a sensor for detecting temperature information, which is arranged on a substrate constituting the body of the positioner 58. A compressed air supply, a source of control air, for activating the air actuator 53, is supplied from the outside to the body of the positioner 58 and a valve setting value is sent through communication from a remote controller. Also, a position signal in accordance with the valve position of the flow control valve 51 is inputted to the body of the positioner 58 outputted from the angle sensor 56.
The operation of the conventional flow control valve will now be described.
The body of the positioner 58 compares the position signal in accordance with the valve position of the flow control valve 51 detected by the angle sensor 56 with the valve opening setting value provided from the outside. Depending on the compared results, a control air signal generated from the compressed air is supplied to the air actuator 53 to drive the valve driving shaft 54 by the air actuator 53 to control flow of a fluid through the valve 51, by controlling the valve position of the flow control valve 51 in order that it coincides with the valve opening setting value given from the outside.
In case a temperature of the fluid to be flow controlled by the flow control valve 51 is greatly different from the normal temperature, the temperature of the body of the positioner 58 and the angle sensor 56 will presents a great difference from the normal temperature as a consequence of heat conduction from the flow control valve 51 through the yoke 52. Therefore, the output of the angle sensor will be varied due to a change in temperature characteristics of the magnetic resistive elements constituting the angle sensor 56. For this reason a temperature characteristic compensating calculation is performed for compensating variations in the output of the angle sensor 56, based on a signal detected by the temperature sensor 59, such as a thermistor mounted on the substrate of the body of the positioner 58. For example, the amplifier for magnetic resistive elements is applicable for this purpose, which is disclosed in Unexamined Japanese Patent Application No. 11-194160. In this case, because the body of the positioner 58 and the angle sensor 56 are housed in the same case, it may be considered that the temperature within this case is uniform. Additionally, the temperature sensor 59 is also used for temperature compensation of circuit elements mounted on the body of the positioner 58.
In the conventional flow control valve, the body of the positioner 58 and the angle sensor 56 are housed in the same case and the case is fixed to the yoke 52. Hence, the temperature sensor 59 is indispensable to a measurement of temperature within the case. However, there is a demand to separate the body of the positioner 58 from the angle sensor 56 and mount it to another place depending on mounting environments. Where the body of the positioner 58 is separated from the angle sensor 56, the temperature sensor 59 will detects only the temperature of the substrate of the body of the positioner 58, and the detected temperature cannot by any possibility be regarded as a temperature of the angle sensor 56 mounted on the another place.
Thus, in the structure where the body of the positioner 58 is separated from the angle sensor 56, another temperature sensor (not shown) similar to the temperature compensating sensor 59 has to be mounted in the vicinity of the bridge circuit consisting of the magnetic resistive elements for the angle sensor 56. As a result, the positioner must have a cable for conducting a position signal in accordance with the valve position of the flow control valve 51 outputted from the angle sensor 56 to the body of the positioner 58, and a separate cable for conducting temperature information obtained in the vicinity of the angle sensor 56 outputted from the newly provided temperature sensor, which complicates its structure.
Also, it has been popular to perform digital signal processing in the body of the positioner 58, by converting analog signals, such as a position signal sent from the angle sensor 56 or a temperature information signal sent from the temperature compensating sensor 59, into digital signals by an A/D converter. Multiple input signals to be A/D converted requires an A/D converter with many channels and results in increased cost.